1. Field of the Invention
The present invention relates to a magnetographic printing process which enables the production of images in color on a print carrier, and a machine for carrying out the process
2. Description of the Prior Art
Magnetographic printing machines which, in response to signals received, which originate from a control unit, enable images, e.g., character images, to be produced on a print carrier generally consisting of a paper strip or sheet are known to those skilled in the art. In such printing machines, which may be of a type similar to that described in French Pat. Application No. 2,305,764, corresponding to U.S. Pat. No. 3,945,343 the images are printed, first, by producing from the signals received a latent magnetic image on the surface of a magnetic recording element. The recording element generally is in the shape of a rotating drum or endless belt. The image consists of a group of magnetized zones of very small dimensions. This latent image is then developed by depositing on this surface a powdery developer containing magnetic particles, which remain applied only to the magnetized zones of the recording element so as to produce a powdery image on the surface of that element. Thereupon, the powdery image is transferred to the print carrier.
For certain special applications it may be desirable for the image thus produced to appear on the print carrier in several different colors. In a previously known process, more particularly as described in French Pat. No. 1,053,634, corresponding to U.S. application Ser. No. 221,044, now U.S. Pat. No. 2,826,634 of M. Ralph Blaisdell Atkinson, filed Aug. 14, 1951, a color image is printed on the print carrier by, first, producing on the recording element a latent magnetic image corresponding to the portions of the same color of the image to be printed, developing this latent image by means of a developer of the same color, transferring onto the print carrier the powdery image thus obtained, and repeating this operation as many times as there are colors in the image to be printed. Such a process, however, is obviously inconvenient because it takes a very long time to carry it out. Furthermore, despite all the care taken in centering the various powdery images during their transfer onto the print carrier, it is virtually impossible to prevent shiftings, however slight, from occurring between the different parts of the image thus printed which, of course, deleteriously affects the definition of the image eventually produced on the print carrier.
To overcome the above drawbacks a magnetographic printing process has been proposed, which is described in U.S. Pat. No. 3,965,478. It consists of producing on the surface of the recording element a large number of magnetized elementary zones, all of which produce a latent magnetic image. Each of these elementary zones is obtained by energizing a recording magnetic head by means of an electric current having a frequency which is selected as a function of the color to be produced by this elementary zone when it is developed. The dimensions and the magnetic attraction of this elementary zone are, moreover, determined by the value of the frequency employed. In this process, the development of the latent image formed on the recording element is accomplished by means of a single developer containing particles of different colors and sizes. All particles of the same size are, however, of the same color. During the development of the latent image, the particles of a given size (and, hence, of a given color) are attracted preferentially by the elementary zones, whose dimensions correspond to a given attractive force so that each elementary zone, after the development, is coated with particles whose color corresponds to the frequency that has been used to produce that elementary zone.
In order to carry out such a process, it is, however, necessary to use a developer whose particles of different colors and different sizes must be carefully calibrated, with all the particles of the same color being exactly of the same size. In addition, these particles must be so conditioned that they do not agglomerate, lest they cause errors in color shades during the development of the magnetic latent image. Under these conditions, the fabrication of such a developer is particularly time-consuming, delicate, and relatively expensive. Furthermore, since, the elementary zones formed on the recording element are not all of the same size, depending on the color assigned to them, the images or parts thereof with a shade corresponding to elementary zones of large dimensions produce a definition, i.e., a distinctness of outline and detail, not as good as those whose shade corresponds to elementary zones of small dimensions. Finally, while during the development the elementary zones of small dimensions are capable of attracting only the smallest particles of the developer, it is impossible to prevent the elementary zones of large dimensions from attracting not only the large particles of the developer, but also smaller particles, which, of course, causes the colors to change.